A model-based tool for reactor configuration of thermophilic biogas plants fed with Waste Activated Sludge

The aim of this study consists in the individuation of the most appropriate configuration for the thermophilic anaerobic digestion of Waste Activated Sludge (WAS); to this purpose, the Anaerobic Digestion Model n°1 (ADM1) was adopted. To enhance the energy production, the sludge underwent a thermal pre-treatment. Semi-continuous tests were performed for 281 days, both with raw and pre-treated WAS. The disintegration constant (k_dis) and the maximum acetate uptake rate (km_ac), were calibrated; the latter parameter was increased up to 2.2 KgCOD Kg−1COD d−1 by the pre-treatment. To evaluate the performances of different plant configurations, the specific methane yield and digester productivity were estimated for varying Hydraulic Retention Times (HRTs) and substrate concentrations. The pre-treatment impact on the substrate resulted in the 20% enhancement of the specific methane production for plants run with HRT <10 days; the most proper input concentration consisted in 30 g of Volatile Solids (VS) d−1. This procedure enabled to estimate the most appropriate HRT range for a) preventing the biomass washout (6/7 days for lower and 8 days for higher input concentration) and b) maximizing the methane and energy production for unit of digester volume (10/12 days).